In the prior art, nickel-cadmium cells have been mainly used as secondary cells, especially for memory backup for audiovisual and information devices such as personal computers and VTRs, and for power sources for operating them. Recently, great attention has been paid to the use of non-aqueous electrolyte secondary cells, which have the advantages of high voltage and high energy density and exhibit excellent self-dischargability, as substitutes for the nickel-cadmium cells. Various attempts have been carried out to develop the non-aqueous electrolyte secondary cells, and some non-aqueous electrolyte secondary cells are available on the market. For example, majority of notebook-sized personal computers, cellular phones, and the like is operated by such non-aqueous electrolyte secondary cells.
In these non-aqueous electrolyte secondary cells, carbon (hard carbon or soft carbon) is often used as a material for forming negative electrodes, and various types of organic solvents are used as electrolytes in order to reduce risk caused at the time of forming lithium on the negative electrodes, and to increase the operating voltage.
Further, in non-aqueous electrolyte secondary cells for cameras, alkali metals (especially lithium metal and lithium alloys) and the like are used as the material for the negative electrodes, and aprotic organic solvents such as ester-containing organic solvents are generally used as the electrolytes in view of reducing risk, which is caused when the alkali metals greatly react with water.
Although the performance of non-aqueous electrolyte secondary cells of these types is high, a problem arises in that the high performance cannot be maintained for a long time since the cells easily deteriorate. Thus, there has been a strong demand for the development of a technique which can inhibit deterioration of the non-aqueous electrolyte secondary cells and keep various properties of the non-aqueous electrolyte secondary cells high for a long time.
Further, the non-aqueous electrolyte secondary cells have the following safety problems. One problem is that the alkali metals (especially lithium metal and lithium alloys) used as the material for the negative electrodes in the non-aqueous electrolyte secondary cells are highly reactive with water, and therefore, when water enters the cells which are not completely sealed, there is a high risk of the negative electrode material in the cells reacting with water to generate hydrogen or catch fire. Another problem is that the melting point of the lithium metals is low (about 170° C.), and therefore, when a large electric current flows abruptly when a short-circuit occurs, very dangerous situations, such as the cells becoming abnormally exothermic and melting, are caused. Still another problem is that, as the cells generate heat, the electrolyte containing as a main component the above-mentioned organic solvent vaporizes and decomposes to generate gas, and the cells burst and catch fire due to the generated gas.
In order to solve these problems, for example, a technique for providing a safety mechanism in cylindrical cells has been proposed (Nikkan Kogyo Shimbun, Ltd., Electronic Technology, Vol. 39, No. 9, 1997), in which the safety mechanism is designed as follows. When the temperature of a cylindrical cell rises when there is a short-circuit or an overcharge and the inner pressure thereof is thereby increased, a safety valve starts to operate and, at the same time, electrode terminals are ruptured to thereby prevent excess current in a predetermined amount or more from running through the cell.
However, the safety mechanism does not always work correctly or, that is, the safety mechanism is not always reliable. When the safety mechanism does not work correctly, excess current may flow through the cell, thereby overheating the cell and causing dangerous situations such as ignition of the cell. Therefore, the safety mechanism proposed is still problematic.
In order to solve the above problem, in place of safety measures taken by providing accessory parts such as the aforementioned safety valve, there is a demand for development of a non-aqueous electrolyte secondary cell essentially having high safety.
A first object of the present invention is to provide a non-aqueous electrolyte secondary cell which has excellent resistance to deterioration, in which the interface resistance of the non-aqueous electrolyte is low, and which has excellent discharge properties at low temperatures, while maintaining characteristics and the like required for a cell.
A second object of the present invention is to provide a non-aqueous electrolyte secondary cell which has excellent self-extinguishability or incombustibility, in which the interface resistance of the non-aqueous electrolyte is low, and which has excellent discharge properties at low temperatures, while maintaining characteristics and the like required for a cell.
A third object of the present invention is to provide a deterioration inhibitor for a non-aqueous electrolyte secondary cell which, by being added into the non-aqueous electrolyte in the non-aqueous electrolyte secondary cell, can prevent deterioration of the non-aqueous electrolyte, can lower the interface resistance of the non-aqueous electrolyte, and which can give excellent discharge properties at low temperatures, while maintaining characteristics required for a cell, such as charge and discharge capacity.
A fourth object of the present invention is to provide an additive for a non-aqueous electrolyte secondary cell which, by being added into the non-aqueous electrolyte in the non-aqueous electrolyte secondary cell, can give self-distinguishability or incombustibility and excellent discharge properties at low temperatures to the non-aqueous electrolyte, and which can lower the interface resistance of the non-aqueous electrolyte, while maintaining characteristics and the like required for a cell.